1. Field of the Invention
The present invention relates to a driving method and a control circuit for generating driving pulses for an impact-drive actuator in which a piezoelectric element is used, and also relates to an image-stabilization control circuit for compensating for shaking of an image-capturing device.
2. Description of the Related Art
Piezoelectric actuators that utilize the electrostriction effects of a piezoelectric element have the potential to serve as ultra-miniature actuators and have conventionally been used for, e.g., camera shake compensation and autofocus.
An impact-drive piezoelectric actuator has a drive shaft for using friction to retain an object of movement. The drive shaft is extended and retracted by a piezoelectric element. A drive circuit generates a voltage signal so that the speed of the drive shaft changes between extension and retraction; i.e., a voltage signal such that the drive shaft extends slowly and retracts rapidly, or vice versa, and the drive circuit applies the voltage signal to the piezoelectric element. During rapid movement of the drive shaft, the object of movement slides with respect to the drive shaft due to inertia and remains substantially in position. If the drive shaft is moved slowly, the position of the object of movement is displaced along with the drive shaft by static friction. The object of movement can be displaced in a specific direction with respect to the drive shaft by this extension and retraction.
This displacement can be achieved in principle by applying a voltage signal having a jagged waveform of different rising and falling times to the piezoelectric element. However, the drive signal outputted from the drive circuit need not necessarily have a jagged waveform, and it is known that the displacement described above can be obtained by adjusting the frequency or duty ratio even when the signal has a square waveform; i.e., when the signal is a pulse signal. The movement speed of the object of movement is dependent on the frequency and duty ratio of the drive pulse, and, e.g., the movement direction can be changed by inverting the duty ratio.
Japanese Patent No. 3358418 discloses a driving device for implementing a control so that at the initiation of driving, the time for which the drive-pulse signal is applied to the piezoelectric element is gradually increased, and the driving speed gradually increases; as well as for implementing a control so that the time for which the drive-pulse signal is applied to the piezoelectric element is gradually reduced, and the driving speed gradually decreases.